1. Field of the Invention
This invention relates to extrusion equipment, and more particularly to a method of air-cooling an extruder cylinder, the length of which includes at least one heating-and-cooling zone, as well as to a device for implementing this method.
2. Description of Related Art
In order for plastics, usually in the form of pellets or granules and coming from a hopper, to be worked in an extruder, they are fed in at one end of an extrusion cylinder and are then led all along the cylinder by means of an extrusion screw to emerge in a pasty form through a die mounted at the other end of the cylinder. The plastic material therefore passes from a solid state to a pasty state over a given distance, this transformation of material involving substantial exchanges of heat. In order for transformation of the material to be complete, it must attain a given temperature before passing through the die; for that purpose, the temperature of the cylinder surrounding the screw must be precisely controlled, this temperature depending essentially on the type of plastic being extruded and the type of screw being used. In order to maintain the cylinder at this given temperature, heating elements and cooling devices are disposed along the cylinder. In general, the latter is divided into several successive independent heating-and-cooling zones, whereby a temperature profile along the cylinder is obtained. Before the machine is started up, the cylinder must be heated by means of the heating elements in order to bring it up to its set temperature, whereas thereafter, the kneading and mechanical constriction imposed on the plastic by the screw supply the system with an amount of heat which may be sufficient or even in excess of what is needed to keep the system at the set temperature. It may therefore be necessary to add a cooling system to the cylinder so that, by combining the heating and cooling cycles, the cylinder, and hence the plastic, remains at an approximately even temperature over a given zone of the cylinder.
Various cooling methods and systems have already been proposed, e.g., those using a cooling liquid such as water or oil. The main drawback of these systems resides in the complexity of the cooling circuit, as well as the particular machining of the extrusion cylinder in order to have the cooling ducts pass through it.
Other systems proposed use a stream of air as the coolant. The advantage as compared with the liquid cooling systems is the simplicity of the circuit and of the cylinder. Such a prior art system for cooling by air circulation is illustrated in FIG. 1, which is a cross-section through the cylinder 10 of an extruder 1. Cylinder 10 comprises a cylindrical interior space in which the screw (not shown) rotates. The cooling circuit 2 is composed, firstly, of a cool-air supply, here represented by a blower 20 causing air, generally drawn from the ambient atmosphere or from a cool-air source, to circulate within an enclosure 21 which completely surrounds the relevant heating-and-cooling zone of cylinder 10 over its entire length. Several systems such as the one shown here are usually installed, one after another all along cylinder 10, each of them constituting a heating-and-cooling zone. Heat-exchange means 22 surround cylinder 10, with various designs such as copper tongues 22A, disks 22B, or wedges 22C being shown in the drawing. Heat-exchange means 22 are associated with the heating circuit 3, heating sleeves 30 of which are merely indicated diagrammatically. As may be seen in this drawing, the circulation of cooling air takes place with a rising movement, coming from blower 20, circulating on both sides of cylinder 10 while passing through heat-exchange means 22, and escaping through a grid 23 or openings made at the top of enclosure 21, as indicated by arrows.
Owing to the rising movement of the cooling air, due particularly to the chimney effect, cylinder 10 is not cooled homogeneously; for the air in contact with the lower part of cylinder 10, designated as region 11, is still cool, whereas it warms up while passing over the sides of cylinder 10 to attain its maximum temperature toward the top of cylinder 10, i.e., the region designated 12. Inasmuch as the air is warmest at the top of cylinder 10, region 12 is less efficiently cooled than region 11; this leads to deformation of cylinder 10 since the elongation of the top part 12 of cylinder 10 is greater than that of the bottom part 11. In view of the longitudinal curvature taken on by cylinder 10, it is no longer exactly coaxial with the screw; the slight clearance existing between the screw and the inside surface of the cylinder is undesirably modified, which leads to premature wear and tear on the screw and on the cylinder, as well as to irregular working of the plastic being kneaded.
Another cooling device is disclosed in U.S. Pat. No. 4,763,722. As in the device described above, the air circulates in a direction perpendicular to the axis of the extrusion cylinder; and although in this case the air is returned for a second passage in the opposite direction, the movement of air always remains perpendicular to the axis of the cylinder, bringing about unequal cooling of the top and bottom parts of the cylinder.